Sintered vitreous silica is a refractory material obtained by sintering amorphous silica powder that has physical and chemical characteristics that make its application of particular interest for numerous industrial uses, notably for the metallurgy of nonferrous metals such as aluminium, zinc, brass, etc.
In effect, this material has a very low coefficient of thermal conductivity (0.7 W/m.K at 700.degree. C.). This characteristic offers significant advantages. When the refractory element is used for the transport or transfer of a molten metal, the heat losses by conduction through the wall of the receptacle or the transfer tube are reduced to the minimum possible. Thus, it is not necessary to overheat the metal (i.e., heat is to a temperature higher than its melting point) to a considerable degree. This results in energy savings.
On the other hand, vitreous silica has a very low coefficient of thermal expansion (0.6.times.10-6 m/K) when the product contains no crystalline phases. This low thermal expansion gives it an excellent resistance to thermal shocks. This makes it possible in most applications to use or to install a refractory element of vitreous silica without having to preheat it beforehand to bring it to a temperature close to its operating temperature. This results in a time gain and also an energy saving. This low thermal expansion permits thermal cycling a large number of times without mechanically fatiguing the material. It also assures a dimensional stability of the product with respect to temperature.
Furthermore, vitreous silica offers a good chemical stability with respect to almost all the materials of molten metals and alloys.
This is why, among other applications, this material is used in nonferrous metallurgy. For example, low-pressure foundry installations are known, in which a mold is fed with metal through its lower part via a tube dipping into a vat filled with molten metal. The pressure of a pulsating gas, e.g., nitrogen or other neutral gas, is applied to the surface of the molten metal to cause it to rise into the mold. Vitreous silica is perfectly suited for the material of the dipping tube.
However, vitreous silica is corroded under the action of a metal that has a greater affinity relative to oxygen. This is the case, for example, of aluminium brought to its melting point or a higher temperature. This metal reduces silica to form a deposit of alumina, leaving metallic silicon. After a period of time that is a function of the thickness of the refractory element, this corrosion ends in the cracking or rupture of the said element. This phenomenon is observed for all applications where the vitreous silica refractory is in contact with the molten aluminium.
A refractory composition resistant to molten aluminium has also been disclosed (U.S. Pat. No. 4,992,395). This composition includes 10-18 wt % ceramic fiber. By ceramic fiber it is meant amorphous or non-crystalline fiber composed of 50% alumina and 50% silica. Such a two phase material is essentially an insulating material suitable for making coatings. Its insulating properties are obtained by the low packing density caused by the fibers during the shaping process. This leads to a porosity of at least 20% which results in the insulating properties of the material. However, such a material does not have enough mechanical resistance to make structural pieces out of it, that is pieces having a mechanical resistance by themselves, and not simply to form the coating of another structure.